Hybrid Warbler (Mniotilta x Vermivora -or- Mniotilta x Dendroica)

 

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On May 14, 2005 at Metro Beach Metropark, Macomb County, Michigan, I observed, tape recorded,  then captured and banded an unusual hybrid warbler. It was singing, thus was likely a male, and a total of 15 photos were taken, all displayed below. Comments were solicited from various internet chat groups, including BIRDBAND and BIRD ID FRONTIERS. 

There seems to be general agreement that one parent is Black-and-white Warbler (Mniotilta varia), while opinions on the other parent are mainly split between Chestnut-sided (Dendroica pensylvanica) and Golden-winged (Vermivora chrysoptera), with "Brewster's" running a distant third. Other possibilities suggested have included Black-throated Green (Dendroica virens) and Black-throated Blue (Dendroica caerulescens), and one suggestion was that this may be a pure Black-and-white Warbler showing some leucism and/or xanthochroism.

Based on breeding habitat in Michigan, it is perhaps more likely for a Black-and-white Warbler to pair with a Chestnut-sided Warbler, and indeed the back pattern is suggestive of Chestnut-sided, as is the wing pattern. But, the buzzy song is more reminiscent of Golden-winged (and Brewster's) Warbler in quality, which breeds in adjacent habitat to Black-and-white and though perhaps more difficult for the two to pair, seems an equally  likely scenario.

Measurements were as follows:

Wing Chord: 65.53 mm
Tail: 50.5 mm
Primary Projection: 14.0 mm
Exposed Culmen: 11.87 mm
Bill Depth at front of Nares: 3.20 mm
Bill Width at front of Nares: 3.52 mm
Tarsus: 17.44 mm

I wish to thank all who provided input on this interesting bird, and am still soliciting opinions and commentary. The photos are numbered to the left of each photo for ease of reference. I also wish to thank Barb Beck for cleaning up my sound recording, and for creating the Sonograms (click here to go directly to the recordings at the bottom of the page)
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Note the head pattern similar to Mniotilta, but with whitish forecrown, and yellow wing bars.

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Directly from the side, the bill was most like Mniotilta, perhaps longer, but clearly decurved and pointed.
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The white-edged black throat feathers are similar to some second-year male Black-and-white Warblers. Note that the angle of this photo gives the bill a more Dendroica-like appearance, an impression that was not noted on the live bird in-hand.
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Another photo from slightly below, showing an almost Dendroica-like appearance to the bill, not noted in the field, and the mottled black throat. The gray cheek patch is most similar to adult female or juvenile male Black-and-white Warbler, or adult female Golden-winged Warbler.
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Again, in the field, the bill and head pattern was very like Mniotilta.
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A closer view of the throat pattern. Also note the thin and tapered bill.
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From above, the back pattern appeared somewhat similar to that of Chestnut-sided, which has green feathers with large black centers, where this bird has gray feathers with large black centers. The black upper tail coverts may be more like Mniotilta. The whitish forehead and crown patch is similar to both Golden-winged and Chestnut-sided in shape and extent. Note the two broad black stripes on the hindcrown and nape, split by a narrow gray median stripe, suggestive of Mniotilta.
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A closer look at the head and back pattern.
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One feature that was visible on the bird in-hand, but which does not show up in any of the photos, is a single pale yellow feather in the center of the whitish crown.
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The tail from below showed fairly large white spots. The rather pointed rectrices suggest perhaps this is a second year bird, but given that this is a cross-genera hybrid, I'm reluctant to use this as a definitive aging criteria.
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From above, the pattern of white on the outer rectrices  is perhaps most like Mniotilta.
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Another view of the tail from above, with the feathers in a more natural position.
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This is the tail of an adult female Golden-winged Warbler for comparison. I do not have a similar photo of adult male, nor of Chestnut-sided or Black-and-white. Such photos could be instructive.
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The open wing, showing details of the greater and median secondary coverts which form the two yellow wing bars. Note the single white tip on the innermost median covert, and some suggestion of whitish on the outermost greater coverts. Also note the broad yellowish outer edges of the greater coverts, which may be suggestive of Golden-winged, though see comparison photos 16 and 17 below.
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This Chestnut-sided Warbler (probably second year) wing shows a superficially similar pattern to the hybrid, but the outer edges of the greater secondary coverts are edged more narrowly with greenish-yellow, and there is less yellow overall in the wing, so is not a perfect match with the hybrid, but is pretty close.
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This Golden-winged Warbler (second year) wing shows a pattern rather unlike the hybrid. I do not have a similar photo of an adult male Golden-winged wing, but it is similar to this but with more extensive yellow on the coverts.
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Just to be thorough, I photographed the birds feet as well. The legs were dark grayish-brown, and the feet were orangish. Offhand, I do not recall the leg and feet colors of any of the potential parent species. The legs were sturdy, and the bird was a larger warbler, requiring a size 0 band (using leg gage), rather than a size 0A taken by smaller-legged warblers (like Golden-winged and Chestnut-sided).
Sound Recordings

Click HERE to listen to the original of a single song (song 6 of 6) recorded of the bird singing. 
Click HERE to listen to a filtered version of the same song (6 of 6). 
Click HERE for a raw version of a different song (2 of 6) from the series (much larger file). 

The bird foraged in a willow tree about 20-30 feet above the ground in a manner similar to a Dendroica or Vermivora, and not using larger branches in the manner of a Mniotilta.

Here is a sonogram of the filtered recording (6 of 6) included here.

 

Here is an additional sonogram made from a different song (4 of 6)